The ITU has defined in ITU-T G.709, in its latest version G.709 (02/12), the signals format and interfaces of the Optical Transport Network (OTN). The basic frame structure is an Optical Transport Module of size k (OTUk), where k can be 1, 2, 2e, 3, 3e2, or 4, where OTU2e and OTU3e2 are described in ITU-T supplement G.sup43 using the principles of ITU-T G.709. It contains framing and section overhead plus a bit-synchronously mapped transport entity termed Optical Data Unit of size k (ODUk). An ODUk contains a payload area plus ODUk overhead. An Optical Payload Unit (OPUk) is mapped into the payload area and carries a client signal or other lower order ODUs being time-division multiplexed.
In an Optical Transport Network, connections are switched on ODU level. The ODU is thus the switchable transport entity that travels along a network path. A network path can be represented by an ODUk. Here two possibilities are possible a direct LO ODUk without substructure which is directly carrying a client or more commonly a higher order (HO) ODUk which will carry multiple lower order (LO) ODUj, which each represent a network path.
While traditionally, each type of ODU had a certain nominal size, a ODU of flexible size termed ODUflex was later introduced to support a larger variety of client signal rates. This could be direct constant bitrate (CBR) signal clients resulting in particular bitrate ODUflexes with particular client adaptations or for the case of Packet data transport packet data commonly encapsulated into GFP frames. A HO OPUk is subdivided into a number of 1.25G tributary slots where an ODUflex can allocate any number of tributary slots. The selection of tributary slots is arbitrary, though all tributary slots should be part of the same ODUk. The size of the GFP mapped data ODUflex can thus be configured in granularity of 1.25G tributary slots up to the maximum number of tributary slots supported by the server ODUk.
To support variable traffic load situations, ITU-T G.7044 (10/11) defines a hitless resizing option for ODUflex connections during operation. The resize protocols defined through G.7044 mainly consists of two parts, the Link Connection Resize (LCR) protocol part and the Bandwidth Resize (BWR) protocol part which are involving section and ODU path layer, respectively.
As in all OSI Layer 1 transport networks, an important feature in OTN is end-to-end protection of network paths. In order to protect an active connection against failures, a redundant protection path is established, where in the case of a failure traffic is switched over from the failed active path to the protection path. Linear protection mechanisms for OTN are defined in ITU-T G.873.1 (07/11) and ODUk shared ring protection is defined in ITU-T G.873.2 (04/12).
However, a protection of ODUflex containers that are resized is not defined and supported in these standards. The problem lies in the complexity of the two part resize mechanism that involves section and path layers and the fact that for a protection to be operable, working and protection connections must have the same size and therefore would have to be resized at the same time, which is not easily possible.
In the recent contribution No. 2254 made to ITU SG15 by J. Youn et al from the Korean's Electronics and Telecommunications Research Institute, it is proposed that the working ODUflex is resized with the current two-step resizing mechanism, while the protection transport entity performs only link connection resizing without ODUflex bandwidth resizing. A new signal bit shall be defined to suppresses the bandwidth resizing process.